Optical apparatuses may change their imaging properties for a wide variety of reasons in the course of their operation. By way of example, material properties may change as a result of aging, and deformations of the optical elements may occur as a result of temperature gradients or as a result of the influence of mechanical forces, for example as a result of the mount. If optical apparatuses are exposed to intensive radiation over a relatively long time, this may likewise lead to alterations of the imaging properties.
On short time scales (seconds), too, for example due to alteration of the quantities of light radiated in (change in the illumination settings or change in the imaging patterns brings about different light paths and intensities in the system), alterations of the imaging properties in the seconds range can be expected.
This relates particularly to projection exposure machines for microlithography. Besides the aging phenomena mentioned, a change in the refractive index may occur, for example, in optical elements made of quartz, as are often used at the present time, under intensive UV radiation loading. Coatings of the optical elements in an objective of a projection exposure machine may also change their properties under irradiation or on account of contamination. All these influences contribute to changing the imaging properties in the course of the objective's operating period. This often gives rise to new image aberrations or amplifies image aberrations that were already present. In objectives for microlithography, therefore, the expected changes in the imaging properties are taken into consideration as early as during the design of the optical system.